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Module Gc

module Gc: sig .. end

Memory management control and statistics; finalised values.

type stat = {
   minor_words : float; (*

Number of words allocated in the minor heap since the program was started. This number is accurate in byte-code programs, but only an approximation in programs compiled to native code.

*)
   promoted_words : float; (*

Number of words allocated in the minor heap that survived a minor collection and were moved to the major heap since the program was started.

*)
   major_words : float; (*

Number of words allocated in the major heap, including the promoted words, since the program was started.

*)
   minor_collections : int; (*

Number of minor collections since the program was started.

*)
   major_collections : int; (*

Number of major collection cycles completed since the program was started.

*)
   heap_words : int; (*

Total size of the major heap, in words.

*)
   heap_chunks : int; (*

Number of contiguous pieces of memory that make up the major heap.

*)
   live_words : int; (*

Number of words of live data in the major heap, including the header words.

*)
   live_blocks : int; (*

Number of live blocks in the major heap.

*)
   free_words : int; (*

Number of words in the free list.

*)
   free_blocks : int; (*

Number of blocks in the free list.

*)
   largest_free : int; (*

Size (in words) of the largest block in the free list.

*)
   fragments : int; (*

Number of wasted words due to fragmentation. These are 1-words free blocks placed between two live blocks. They are not available for allocation.

*)
   compactions : int; (*

Number of heap compactions since the program was started.

*)
   top_heap_words : int; (*

Maximum size reached by the major heap, in words.

*)
}

The memory management counters are returned in a stat record.

The total amount of memory allocated by the program since it was started is (in words) minor_words + major_words - promoted_words. Multiply by the word size (4 on a 32-bit machine, 8 on a 64-bit machine) to get the number of bytes.

val stat : unit -> stat

Return the current values of the memory management counters in a stat record. This function examines every heap block to get the statistics.

val quick_stat : unit -> stat

Same as stat except that live_words, live_blocks, free_words, free_blocks, largest_free, and fragments are set to 0. This function is much faster than stat because it does not need to go through the heap.

val counters : unit -> float * float * float

Return (minor_words, promoted_words, major_words). This function is as fast at quick_stat.

val minor : unit -> unit

Trigger a minor collection.

val major_slice : int -> int

Do a minor collection and a slice of major collection. The argument is the size of the slice, 0 to use the automatically-computed slice size. In all cases, the result is the computed slice size.

val major : unit -> unit

Do a minor collection and finish the current major collection cycle.

val full_major : unit -> unit

Do a minor collection, finish the current major collection cycle, and perform a complete new cycle. This will collect all currently unreachable blocks.

val compact : unit -> unit

Perform a full major collection and compact the heap. Note that heap compaction is a lengthy operation.

val print_stat : Stdlib.out_channel -> unit

Print the current values of the memory management counters (in human-readable form) into the channel argument.

val allocated_bytes : unit -> float

Return the total number of bytes allocated since the program was started. It is returned as a float to avoid overflow problems with int on 32-bit machines.

val finalise : ('a -> unit) -> 'a -> unit

finalise f v registers f as a finalisation function for v. v must be heap-allocated. f will be called with v as argument at some point between the first time v becomes unreachable and the time v is collected by the GC. Several functions can be registered for the same value, or even several instances of the same function. Each instance will be called once (or never, if the program terminates before v becomes unreachable).

The GC will call the finalisation functions in the order of deallocation. When several values become unreachable at the same time (i.e. during the same GC cycle), the finalisation functions will be called in the reverse order of the corresponding calls to finalise. If finalise is called in the same order as the values are allocated, that means each value is finalised before the values it depends upon. Of course, this becomes false if additional dependencies are introduced by assignments.

Anything reachable from the closure of finalisation functions is considered reachable, so the following code will not work as expected:

Instead you should write:

The f function can use all features of O'Caml, including assignments that make the value reachable again. It can also loop forever (in this case, the other finalisation functions will be called during the execution of f). It can call finalise on v or other values to register other functions or even itself. It can raise an exception; in this case the exception will interrupt whatever the program was doing when the function was called.

finalise will raise Invalid_argument if v is not heap-allocated. Some examples of values that are not heap-allocated are integers, constant constructors, booleans, the empty array, the empty list, the unit value. The exact list of what is heap-allocated or not is implementation-dependent. Some constant values can be heap-allocated but never deallocated during the lifetime of the program, for example a list of integer constants; this is also implementation-dependent. You should also be aware that compiler optimisations may duplicate some immutable values, for example floating-point numbers when stored into arrays, so they can be finalised and collected while another copy is still in use by the program.

The results of calling String.make, String.create, Array.make, and Stdlib.ref are guaranteed to be heap-allocated and non-constant except when the length argument is 0.

val finalise_release : unit -> unit

A finalisation function may call finalise_release to tell the GC that it can launch the next finalisation function without waiting for the current one to return.

type alarm

An alarm is a piece of data that calls a user function at the end of each major GC cycle. The following functions are provided to create and delete alarms.

val create_alarm : (unit -> unit) -> alarm

create_alarm f will arrange for f to be called at the end of each major GC cycle, starting with the current cycle or the next one. A value of type alarm is returned that you can use to call delete_alarm.

val delete_alarm : alarm -> unit

delete_alarm a will stop the calls to the function associated to a. Calling delete_alarm a again has no effect.